The present invention relates generally to detection systems as disclosed in co-pending application Ser. No. 909255, filed May 24, 1978, entitled SYSTEM FOR DETECTING PARTICLES OR VOIDS IN PLASTIC MATERIAL AND METHOD (assigned to the assignee of the present application) and more particularly to a system for detecting foreign particles or voids in the outer insulation jacket of an electrical cable, specifically a power cable including a cylindrical insulation jacket constructed of polyethylene material having a predetermined index of refraction and absorption coefficient in a preferred embodiment.
It is well recognized that underground cables for power transmission require insulation coatings which must not only have good electrical properties but also good physical properties so that the cable can withstand the rigors of an underground installation operation and varied environmental conditions in the subsurface area. Commonly used insulating materials include solid plastics such as polyethylene which has many desired physical properties, such as outstanding moisture resistance, high dielectric strength, and extremely low loss characteristics, in addition to many other desired properties.
The use of polyethylene as an extrudable dielectric in high voltage power cables was first recognized in 1945. This recognition occurred as a result of the early usage of polyethylene in voltage coaxial transmission cables for radar systems during the war. The inherent low loss electromagnetic characteristics and high dielectric strength of polyethylene was found to be far superior to that of butyl and oil base compounds. Due to these excellent characteristics, polyethylene has now found extensive usage as the insulator on transmission line cables which are to be buried under ground and operated in excess of 65 kV. The adverse underground environment in conjunction with the high installation expenses in burying the cables, places high and difficult quality control standards on the manufacturing of these cables. At the present time, the desired quality control throughout the industry exceeds the capability of the presently available technology to measure the quality of the product as it is coming off the manufacturing line. This is particularly true of the detection of tiny voids (i.e. bubbles) during the extrusion process. In this regard, the particular polyethylene utilized in underground high voltage cables is especially susceptible to the formation of such voids during the extrusion process and specifically upon cooling. These voids can be electro-active and cause interference as well as premature failures. Such failures are particularly bothersome on high voltage cables which are to be buried underground due to the safety aspects involved and the high cost which is involved in retrieving the cable, repairing it and burying it again.
In the early days (prior to 1948) detection of these voids was crudely accessed by plotting 16 Hz power factor readings against applied voltage and noting the break in which ionization occurred. In 1948, the first Corona level detector was introduced by which the relationship between voids and cable life was established. During the last 20 years, extensive efforts were devoted to the development of a partial discharge detection system by numerous manufacturers. This system is now capable of detecting a single void as small as two (2) mils in diameter with a sensitivity of one (1) picocoulomb on 5000 foot cable lengths. However, detection of voids utilizing this particular system is limited to "clean" voids, which have no foreign gas or other materials inside them.
Recent investigations have since disclosed that the majority of voids appearing in underground high voltage cable during the manufacturing process are not clean under atmospheric pressure. However, as just stated, in the partial discharge detection system the void is required to be clean in order to be detectable, that is, in order for it to break down under applied 60 Hz high voltage and thus generate high-frequency signals capable of detection by present radio techniques. These "unclean voids" also called "internally shielded voids," are equally responsible as clean voids for premature cable failures, particularly failures relating to what is referred to as a "electrochemical treeing phenomena."
It has been established that the typical process for manufacturing this high voltage cable and its environment are in large part responsible for this transformation of clean voids to internally shielded voids, that is, "unclean voids." There are a number of factors which contribute to this conclusion. First, the present manufacturing process relies on water cooling or cross-linking under steam pressure which allows vapor transmission to occlude the void. Second, byproducts given off during the chemical cross-linking process can occlude the void by filling it with a high-pressure gas or a chemical liquid. Finally, permeation or expulsion of water vapor from the voids under low cycling of the cable after installation thereof can also lead to failures.
In addition to the fact that partial discharge detection is not completely satisfactory when required to detect unclean voids, recent studies have indicated that even the most sensitive instruments presently available to underground cable manufacturers are not capable of detecting micro-porosity in the order of two (2) to five (5) micrometers. These micro-porosity voids are not only susceptible to electrochemical treeing, but also precludes cable design and fabrication utilizing the ultimate inherent dielectric break-down capabilities that could be achieved with polyethylene. Furthermore, partial discharge detection can only be conducted as a final test on shielded cable ready for shipment. Hence the presently available instrumentation is not capable of in-process control and considerable scrap results during the manufacturing process because most extruder runs are in the order of 100,000 feet.
As will be seen hereinafter, the present invention is directed to a system for detecting tiny voids as well as contaminants (e.g. foreign particles or unclean voids) and particularly to an in-line system for continuously monitoring the polyethylene insulation jacket as it is extruded into its final shape. In this regard, the presence of solid contaminants contributes equally to premature cable failure and electrochemical treeing, particularly when contaminant boundary does not "wet" the insulation and forms a void therein. Contaminants cannot be detected by the partial discharge measurement techniques (as they are characterized as unclean voids) and the manufacturer must rely on visual assessment by microscopic analysis of, for example, a two-inch specimen for each 10,000 feet of cable, as required by present industry standards. This sample inspection also results in large scrappage and unfortunately it cannot assure the absence of contaminants in the remaining unexamined cable length.
The need for an advancement in the state-of-the-art of the detection of voids and contaminants, regardless of size and internal occulusions, is apparent and must be recognized if the desired service life and ultimate design of polyethylene insulated cable is to be achieved. As stated above, the present invention is directed towards a system capable of in-process detection of voids and contaminants during the manufacturing process without any of the limitatons of the detection systems presently employed. This system uses a laser beam of electromagnetic energy, specifically far-infrared radiation in a preferred embodiment. As will be seen hereinafter, in this preferred embodiment versatility comes mainly from the fact that a host of far-infrared laser transitions are available and can be used to match the void or contaminant sizes of interest. Moreover, it has been specifically found that laser radiation scattering techniques in the far-infrared band is extremely sensitive and permits the detection of one single particle or void in the polyethylene jacket. However, as will also be seen, the system which accomplishes this, in its preferred embodiment, is relatively uncomplicated in design and reliable in use.